1. Field of the Invention
The present invention relates generally to apparatus for rapidly and accurately measuring the flexural creep compliance, or creep stiffness, of asphalt cement, polymers, and other viscoelastic materials at temperatures at which they exhibit relatively high stiffness, generally greater than 1,000 psi.
2. Description of the Prior Art
The mechanical properties of some viscoelastic materials, that is, materials having some elasticity and slow flow or creep, cannot be determined by customary methods of shear rheometry. Their peculiar properties rather involve factors requiring a very considerable time element, as opposed to the rapid type of testing using shear rheometers. Also, there is a temperature factor which enters in the case of certain materials. One illustrative instance is the comparative measurement of asphalts as applicable to a determination of their characteristics for service life in pavements and the like. For instance, asphalt cement in pavement usage is subjected to stresses more particularly brought about by thermal expansion and contraction as caused by ambient temperature changes, freezing and thawing and the like. Hot-mix asphalt pavements are particularly characterized by their ability to undergo deformation, for example, to bend without breaking, in transmitting traffic loads to the foundation and natural sub-grade. In one sense, therefore, hot-mix asphalt pavements must be characterized by their response to stresses to include the effect of certain long-time factors affecting deformability.
Isolated properties such as penetration, softening point, and the like, as commonly determined for asphalt cements, are not sufficiently informative as to the overall behavior which may be expected of a given material in actual service life. Such fundamental properties as viscosity and elasticity enter, but extensive investigation has shown that these fundamental variables may change counter to each other with changes in temperature, and other environmental conditions, such that these individual characteristics are by themselves, of limited value in judging relative behavior of different materials.
Load-deformation response as a function of loading time and temperature, however, embodies the essential factors to such an extent that measurement of deformability becomes more convenient and practical as a basis of comparison. The present invention accordingly provides means by which deformability, at low stress levels, of asphalt, and materials generally which exhibit slow flow characteristics, may be relatively evaluated in a practical manner. One of the most important criteria of usefulness is the amount of cold flow under stress, commonly called creep. In the common forms of creep testing, the specimen is subjected to a torsion or shear load over a prolonged period of time. At various times during the period of loading, the deflection of the material is measured and subsequently plotted against time to serve as a measure of the fundamental engineering response of the material.
Determination of the creep compliance of asphalt cement at low temperatures is extremely important, since many pavement failures are caused by excessive stiffness of the asphalt binder at low temperatures. Previously, there was no simple and accurate means of determining the stiffness of asphalt cement at low temperatures. It was with knowledge of the existing situation that the present invention was conceived and is now reduced to practice.